1. Field of the Invention
The subject invention is directed generally to a scientific laboratory analytical equipment, and more particularly, to analytic equipment with a closed loop feedback controller for a high pressure pump.
2. Background of the Related Art
Scientific laboratories commonly need to separate chemical compounds on such basis as the compounds molecular weight or size, charge or solubility. Separation of the compounds is often a first step in the identification, purification and quantification of the compounds. Chromatography or, more specifically, high performance liquid chromatography (BPLC) has become the analytical tool of choice for applications as varied as biotechnological, biomedical, and biochemical research as well as for the pharmaceutical, cosmetics, energy, food, and environmental industries.
As advances in technology emerge, manufacturers of HPLC instruments are quick to improve the performance of their product lines. In fact, improvements in one technological area or subsystem typically spurn on advancement in interrelated areas or subsystems. U.S. Pat. No. 6,187,595 to Staal, which is incorporated herein by reference in its entirety, discusses several advantages and disadvantages related to evolving approaches based on new technology.
Currently, there are several types of pumps commonly used as subsystems with HPLC instruments. HPLC instruments may incorporate reciprocating pumps, syringe pumps, and constant pressure pumps as are known to those of ordinary skill in the art. For example, most reciprocating pumps include a small motor driven plunger which moves rapidly back and forth in a hydraulic chamber to vary the volume thereof. On the back stroke, the plunger pulls in a solvent. On the forward stroke, the plunger of the reciprocating pump pushes the solvent out to a column. In order to achieve the desired flow stability within the column, multiple plungers are employed, normally two. The two plungers may be employed in series or in parallel to achieve the desired delivery flow and pressure.
During compression of the solvent, energy is absorbed that raises the temperature of the solvent. This thermal effect is proportional to the solvent compressibility, the target pressure (e.g., the desired instrument pressure) and the rate at which the solvent is compressed. For many leading edge technology HPLC instruments, the high pressure and limited amount of time to compress the solvent creates significant thermal effect. The heat is usually dissipated to the surroundings and associated instrument at a rate dependent upon the relative mass and thermal conductivity of the compressed solvent and the surroundings. In most applications, for pressures up to a couple thousand psi, the thermal effects of compression are negligible.
However, the thermal effects at high pressure become more appreciable. The thermal effects create errors in the pressure of the compressed solvent because the solvent temperature is elevated during compression compared with during analysis in the instrument. In other words, just after the solvent is compressed to the target pressure, the pressure decays as the solvent temperature moves toward equilibrium with the instrument. As a result, the compressed solvent settles to a pressure below the target pressure and, thereby, creates a deficit in delivered flow.
Prior art pump control systems lack the required ability to react to the thermal effects of solvent compression at high pressures. So despite the advances of the state of the art, HPLC instruments are lacking in stability and performance. As a result, inaccurate results are still common. Moreover, such prior art instruments are plagued by inadequacies such as complex electronics and numerous additional components that undesirably increase costs and complexity without overcoming the noted drawbacks. In view of the above, it would be desirable to provide a controller for a high pressure pump that affords accurate delivery of the target pressure and the ability to compensate for thermal effects.